1. Field of the Invention
The present invention relates to a wireless communication apparatus.
2. Description of the Related Art
A wireless system uses a technique of transmitting a plurality of frames upon aggregating them to improve communication efficiency. For example, Task Group n (TGn), whose standardization has been under way via IEEE801.11, has proposed an aggregation technique of aggregating and transmitting a plurality of frames. This technique of aggregating and transmitting a plurality of frames can reduce overheads such as physical (PHY) and MAC layer headers accompanying each frame, which are required at the time of transmission and reception of frames, and the interval between frames.
On the other hand, as the frame length at the time of transmission and reception increases excessively, the wireless channel state estimated at the head of a frame differs from the state at the rear half portion of the frame, resulting in an error. That is, a proper frame length depends on the state of the wireless channel. There has therefore been proposed a mechanism of controlling the frame length in accordance with the state of the wireless channel by using this characteristic associated with the frame length (see JP-A 2004-260658 (KOKAI)).
For example, the technique in JP-A 2004-260658 (KOKAI) performs control to increase the frame length when the communication rate is low and the error rate is low, and to decrease the frame length when the communication rate is high and the error rate is high.
This technique also enumerates parameters associated with frame length control which decreases the frame length when the retransmission count is close to the maximum value and increases the frame length when the data occurrence frequency is low.
In an actual wireless environment, a transmission rate control, called link adaptation, is performed in accordance with the wireless environment. A conventional technique performs transmission rate control such as selecting a transmission rate with a packet error rate (PER) which satisfies a target error rate on the basis of the relationship between the PER and the target error rate.
When aggregating and transmitting a plurality of frames, as described above, since even the frame length at the time of transmission/reception is associated with the occurrence of errors, it is necessary to simultaneously consider transmission rate control and frame length control for frames to be aggregated. In this case, whether to control the transmission rate or the frame length cannot be determined only by a comparison between the PER of overall frames after aggregation and the target error rate.
If, for example, an error occurs due to the frame length after aggregation, conventional rate control is performed to change the transmission rate even in a wireless environment which requires no transmission rate control, resulting in a decrease in throughput. In contrast, even control on the frame length after frame aggregation in a wireless environment which requires rate control does not contribute to an improvement in error reduction, resulting in a decrease in throughput.
It is therefore ideal to select an optimal frame length and transmission rate by calculating a throughput from the frame length and transmission rate after the aggregation of a plurality of frames. Every time frame length control or transmission rate control is performed in accordance with whether the state of a wireless channel is good/bad, it is necessary to calculate and compare throughputs based on two or more combinations (frame lengths and transmission rates). When a transmission rate x and a frame length y are set and a wireless channel state is good, it is necessary to calculate throughputs with respect to at least combinations of “x and (y+1)” and “(x+1) and y”. Furthermore, this calculation does not take into consideration error proneness in the rear half portion upon aggregation of a plurality of frames.
As a mechanism of giving consideration to error proneness in the rear half portion upon aggregation of a plurality of frames, it suffices to use a technique of holding a PER table corresponding to “selectable rate count (MCS count)×aggregation count×packet length” and select a proper frame aggregation count and transmission rate by referring to the table information. However, since the PER table changes according to the wireless environment, it is not realistic to hold PER tables in various wireless environments.
As described above, when aggregating and transmitting a plurality of frames, the conventional technique cannot easily perform control to increase/decrease the frame length and the transmission rate, in accordance with whether the wireless channel state is good/bad (the error rate of reception frames), on the basis of the throughput.